In recent years, the reduction of CO2 emissions has been sincerely desired in order to address global warming. The automotive industry has a growing expectation on the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) for the reduction of CO2 emissions and has been intensively working on the development of motor-driving secondary batteries, which become key to the practical application of these electric vehicles.
The motor-driving secondary batteries are required to have very high output characteristics and high energy as compared to consumer lithium-ion secondary batteries for mobile phones, notebook computers etc. Attentions are being given to lithium-ion secondary batteries having relatively high theoretical energy among all batteries. The development of such lithium-ion secondary batteries has been pursued rapidly at present.
In general, the lithium-ion secondary battery includes a positive electrode in which a positive electrode active material is applied with the use of a binder etc. onto both sides of a positive electrode collector and a negative electrode in which a negative electrode active material is applied with the use of a binder etc. onto both sides of a negative electrode collector. These positive and negative electrodes are connected to each other via an electrolyte-impregnated separator and accommodated in a battery case.
One of problems for the practical use of the secondary batteries such as lithium-ion secondary batteries is that, for some reason, a so-called internal short circuit occurs between the positive and negative electrode active material layers even through the positive and negative electrode active material layers are separated from each other by the separator. The occurrence of an internal short circuit in the battery leads to the continuous flow of an electric current through a power generating element of the battery even though the battery has no connection to an external load. This becomes a cause of heat generation in the battery and, in some cases, causes an adverse effect on the performance of the battery. It has been demanded to develop techniques for effectively preventing such a problem of internal short circuit and heat generation. Under present circumstances, however, there have not yet been established any adequate solution to the problem of internal short circuit and heat generation. It is necessary in the development of the techniques for preventing the problem of internal short circuit and heat generation not to cause an adverse effect on the performance of the battery in view of the fact that the battery is in need of further performance improvements.
On the other hand, Patent Document 1 discloses a technique for forming a negative electrode active material layer of a lithium-ion secondary battery with the use of a carbon negative electrode active material containing nanoparticles of ceramic material such as alumina. It is conventionally likely that, when the thickness of the negative electrode active material layer becomes increased for improvement in the energy density of the lithium-ion secondary battery, deposition of lithium metal will occur with increase in the charge density of the negative electrode. The technique of Patent Document 1 is intended to solve such a lithium deposition problem and is not intended to prevent the above-mentioned problem of internal short circuit and heat generation.